1. Field of the Invention
This invention relates to a zoom lens which can be continuously focused to objects from infinity to a very short distance.
2. Description of the Prior Art
Generally, photographic lenses are designed with infinity object distance as the standard and therefore, when they are focused to a short distance object, their imaging performance is usually aggravated. Particularly, in zoom lenses, various focusing methods for short distance photography have been proposed to enhance their multi-function, but the deterioration of their imaging performance at short distances is substantial and therefore, even in a lens having a good imaging performance for infinity object distance, aggravation of various aberrations is substantial during photographing of short distance objects and thus, such a lens has not been practical unless it is stopped down considerably. Also, a zoom lens is basically much more bulky in construction than a fixed focus lens having the same focal length as the focal length on the wide side. To enable the zoom lens to be focused to a considerably short distance only by a first group thereof, the aperture of the first group must be made larger and this makes it necessary to give sufficient consideration to the compactness of the lens shape. If the aperture of the lens is made small, the quantity of marginal light is liable to become deficient particularly at short distances and, if the full length of the lens and the focal length of the first group are made short, correction of various aberrations will become difficult and therefore, it has been very difficult to maintain good imaging performance for short distances while keeping the shape of the entire lens system compact.